1. Field of the Invention
This invention relates to a process for the production of dinitrotoluene by a two-stage reaction of toluene with nitric acid in the presence of sulphuric acid, toluene being nitrated to mononitrotoluene in the first stage, using the spent acid from the second stage in which the mononitrotoluene is nitrated to dinitrotoluene, using the spent acid from the first stage concentrated under vacuum.
2. Background Information
The removal or recovery of spent sulphuric acid from the production of dinitrotoluene is a problem in that sulphuric acid constitutes a substantial proportion of the production costs. This has in the past led to attempts to obviate the use of sulphuric acid altogether (Kirk-Othmer, Encycl. Chem. Techn. 3rd Edition, 1981, Volume 15, pages 928-929) and to attempts to make the sulphuric acid usable, by more or less elaborate processes for purification, in the fertilizer industry or render it suitable for recirculation before concentration by evaporation (U.S. Pat. No. 4,257,986).
The most widely used process for concentrating spent sulphuric acid by evaporation is the Pauling process (Bodenbrenner, von Plessen, Vollmuller, Dechema-Monogr. 86 (1980), 197), in which a relatively pure 96% sulphuric acid may be recovered.
The disadvantages of this process reside in the high capital investment and operating costs, as well as the formation of SO.sub.2 and NO.sub.x compounds and gases due to oxidative decomposition of some of the organic compounds.
Substantial removal or recovery of the organic compounds from spent acid containing sulphuric acid may be achieved according to U.S. Pat. No. 3,856,673 by stripping with steam at temperatures of from 130.degree. to 230.degree. C.
The known methods of concentration of sulphiric acid by evaporation under vacuum (Winnacker. Kuchler, Chem. Technol., Vol.2, Anorg. Technol. I, 4th Edition, 1982, pages 70-72) give rise to numerous problems when used for the concentration by evaporation of untreated spent sulphuric acid (hereinafter referred to as "spent" acid") from the production of dinitrotoluene, which has hitherto prevented the use thereof for this purpose. When the toluene nitration process is carried out under optimum conditions, the spent acids contain mainly nitrosyl sulphuric acid, dinitrotoluenes (DNT), mononitrotoluenes (MNT) and nitric acid in addition to water and metal sulphates.
DNT and MNT are volatile in steam and are to a large extent removed by evaporation together with the water. At the low condensation temperatures required for operation under vacuum, DNT crystallizes and causes blockages in the condensation system. The direct condensation of vapors in injection condensers using fresh cooling water is not an economic process because it gives rise to excessive quantities of contaminated waste water and the MNT and DNT contained in the waste acid are lost. If, on the other hand, an indirectly-cooled circulation of vapor condensate is maintained, the solid DNT gives rise to problems similar to those found in vapor condensation on cooled heat exchange surfaces.
It has proved almost impossible to overcome these problems. Removal of the organic constituents by extraction before concentration of the acid by evaporation leads to completely unsatisfactory results. Although the expensive method of steam stripping removes most of the organic compounds, considerable quantities, in particular of 2,4-dinitrotoluene, are left in the spent acid.